In the construction of many buildings, a finished ceiling, which is referred to as a drop ceiling, is spaced below a structural floor panel that is constructed of concrete, for example. Light fixtures as well as other items appear below the drop ceiling. The space between the ceiling and the structural floor from which it is suspended serves as a return-air plenum for elements of heating and cooling systems as well as a convenient location for the installation of communications cables including those for computers and alarm systems. It is not uncommon for these plenums to be continuous throughout the length and width of each floor.
When a fire occurs in an area between a floor and a drop ceiling, it may be contained by walls and other building elements which enclose that area. However, if and when the fire reaches the plenum, and if flammable material occupies the plenum, the fire can spread quickly throughout an entire story of the building and smoke can be conveyed through the plenum to adjacent areas. The fire could travel along the length of cables which are installed in the plenum.
Generally, a cable sheath which encloses a core and which comprises only a vinyl plastic jacket does not exhibit what are now totally acceptable flame spread and smoke evolution properties. As the temperature in such a cable rises, charring of the jacket material begins. Afterwards, conductor insulation inside the jacket begins to decompose and char. If the jacket char retains its integrity, it functions to insulate the core; if not, it is ruptured by the expanding insulation char, exposing the virgin interior of the jacket and insulation to elevated temperatures. The jacket and the insulation char begin to pyrolize and emit flammable gases. These gases ignite and, because of air drafts within the plenum, burn beyond the area of flame impingement, propagating flame and evolving smoke.
In order to thwart flame spread and smoke evolution, the National Electric Code (NEC) requires that power limited cables in plenums be enclosed in metal conduits. The initial cost of rigid metal conduits for communications cables, for example, in plenums is relatively expensive. However, the Code permits certain exceptions to this requirement. For example, flame-resistant, low smoke-producing cables without metallic conduit are permitted, provided that such cables are tested and approved by an authority such as the Underwriters' Laboratories. What is needed for use in buildings is a cable which is relatively inexpensive to manufacture, but which meets the NEC requirements for flame retardance and smoke evolution, and which has suitable mechanical properties such as flexibility.
The prior art includes a cable which comprises a core enclosed in a paper wrap and in a relatively thick metallic shield but it is relatively inflexible and somewhat difficult to maneuver in plenums. Also, care must be taken during installation to guard against possible electrical shock which may be caused by the metallic shield of the above-described cable engaging exposed electrical service wires or equipment. One commercially available fluorine-containing polymer material has been accepted as the primary insulative covering for conductors and as a jacketing material for plenum cable without the use of metal conduit. However, that material has a relatively high dielectric constant which makes it unattractive as insulation for communications conductors.
A plenum cable that has superior resistance to flame spread and smoke evolution is shown in U.S. Pat. No. 4,284,842 which issued on Aug. 18, 1981 in the names of C. J. Arroyo, N. J. Cogelia and R. J Darsey and which is incorporated by reference hereinto. It includes a reflective sheath system which encloses a core and which comprises a layer that is made of a thermal core wrap material and a corrugated metallic barrier having longitudinal edge portions that form a seam. The metallic barrier which reflects radiant heat outwardly is covered with two translucent tapes. Each tape is wrapped helically about the metallic barrier with overlapped sealed seams.
The foregoing sheath system, which depends on its reflection characteristics to keep the heat away from the core is well suited to larger pair size plenum cables. However, for smaller pair size cables such as those containing twenty-five pairs or less, the use of a corrugated metallic shield is not only expensive, but is somewhat difficult to form about the core. Also, inasmuch as the metallic barrier reflects heat, manufacturing line speeds must be low enough to allow sufficient heat energy to be transferred to adhesive on the tapes to seal the seams.
In one prior art small pair size cable, the core is protected by a non-metallic sheath system having a relatively low thermal conductivity. See application Ser. No. 518,582 filed July 29, 1983, in the names of C. J. Arroyo et al now abandoned. The sheath system includes an inner layer of a fibrous material, which has a relatively low thermal conductivity and a relatively high heat absorptivity, and a woven glass layer which is impregnated with a fluorocarbon resin material and which is wrapped helically about the core. Although the sheath system provides excellent results, the fibrous material is difficult to form about a cable having only several pairs of conductors.
What is still sought is a less expensive, flame retardant, smoke suppressive sheath system for a relatively small pair size plenum cable. The sought-after cable desirably is easier to manufacture than presently available products and includes a core wrap material that is capable of being wrapped easily about the core.